This invention relates to query processing in federated database systems. A federated database system is a type of database management (DBMS) system which transparently integrates multiple autonomous database systems, referred to below as source servers, into a single federated database. The source servers are interconnected through a computer network and can be geographically decentralized. Since the constituent database systems remain autonomous, a federated database system is often a viable alternative to merging together several disparate databases.
Through data abstraction, federated database systems can provide a uniform frontend user interface, thereby enabling users and clients to store data in and retrieve data from multiple non-contiguous source servers with a single query, even if the constituent source servers are heterogeneous. The federated database management system receives a query from a user or client that references tables stored and managed by one or more source servers, optimizes the query into subqueries that can be executed by those source servers, and coordinates the execution of the received query by distributing the subqueries to the servers for execution, and by combining subquery results into a result for the received query that is returned to the querying user or client. Some common examples of source servers include the DB2 z Series and the Informix IDS series, both available from International Business Machines Corporation of Armonk, N.Y.
A problem with federated query processing is that data from different source servers must be combined on the federated server. The movement of data from the source servers to the federated server requires the federated database system to do a significant amount of processing, and hence use a commensurate amount of system resources, given that there is likely to be a large amount of data transferred from the source servers. Moreover, the computer network might become a bottleneck due to the large amount of data being moved across the network from the source servers to the federated server. Thus, there is a need for more efficient processing techniques for federated queries.